## TPTP Problem File: LCL181+1.p

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```%------------------------------------------------------------------------------
% File     : LCL181+1 : TPTP v8.1.0. Released v2.0.0.
% Domain   : Logic Calculi (Propositional)
% Problem  : Principia Mathematica 2.15
% Version  : Especial.
% English  : Judged by [SRM73] to be the 'hardest' of the first 52 theorems
%            of [WR27].

% Refs     : [WR27]  Whitehead & Russell (1927), Principia Mathematica
%          : [SRM73] Siklossy et al. (1973), Breadth First Search: Some Sur
%          : [Pel86] Pelletier (1986), Seventy-five Problems for Testing Au
%          : [Hah94] Haehnle (1994), Email to G. Sutcliffe
% Source   : [Hah94]
% Names    : Pelletier 4 [Pel86]

% Status   : Theorem
% Rating   : 0.00 v2.5.0, 0.33 v2.4.0, 0.33 v2.2.1, 0.00 v2.1.0
% Syntax   : Number of formulae    :    1 (   0 unt;   0 def)
%            Number of atoms       :    4 (   0 equ)
%            Maximal formula atoms :    4 (   4 avg)
%            Number of connectives :    5 (   2   ~;   0   |;   0   &)
%                                         (   1 <=>;   2  =>;   0  <=;   0 <~>)
%            Maximal formula depth :    4 (   4 avg)
%            Maximal term depth    :    0 (   0 avg)
%            Number of predicates  :    2 (   2 usr;   2 prp; 0-0 aty)
%            Number of functors    :    0 (   0 usr;   0 con; --- aty)
%            Number of variables   :    0 (   0   !;   0   ?)
% SPC      : FOF_THM_PRP

% Comments :
%------------------------------------------------------------------------------
fof(pel4,conjecture,
( ( ~ p
=> q )
<=> ( ~ q
=> p ) ) ).

%------------------------------------------------------------------------------
```